There are many applications in which it is desirable have a voltage controlled oscillator with low phase noise. In some applications, in order to reduce the number of oscillators required to cover a given frequency band, it is desirable for the oscillator to have low phase noise over a wide tuning range.
Voltage controlled oscillators are known in the art. One conventional voltage controlled oscillator includes an LC resonator consisting of two back to back series varactor diodes and a single resonant inductor. Frequency control is achieved by applying a DC voltage through a bias choke connected at the junction of the varactor diodes common cathode. One disadvantage of this voltage controlled oscillator is that a high frequency voltage applied to the resonator is applied across just two varactor diodes. When attempting to cover a wide frequency bandwidth it is desirable to utilize the full DC control voltage range. Therefore, the DC control voltage may be selected to be a low voltage in the range. At a low DC control voltage the varactor capacitance will change significantly and nonlinearly for a small change in DC control voltage. Then, when a high frequency voltage is applied across the two varactor diodes at this low DC control voltage, it results in a nonlinear modulation of the desired oscillator signal and consequently degraded phase noise. U.S. Pat. No. 4,536,724 relates to a prior art voltage controlled oscillator which includes multiple electrically tunable varactor diodes arranged in a matrix that along with a single inductor forms an LC resonant circuit. An advantage of that oscillator is that a high frequency voltage applied to the resonator is distributed equally among a plurality of series back to back varactor elements resulting in relatively less high frequency voltage applied across each diode in the matrix. One disadvantage of that voltage controlled oscillator is that at high frequencies the parasitic capacitances and inductances associated with the physical realization of the varactor diode matrix greatly reduces its frequency tuning bandwidth.
Further attempts to improve performance have included using resonant circuits with a plurality of series resonators each resonator employing distributed inductance and capacitance. The inductors are often fixed and the capacitances are variable and implemented with e.g. electrically variable capacitances such as varactors. Such devices provide improved results with lower phase noise but still more improvement is desirable.